The present invention relates to an electroacoustic transducer for use in a portable communication device, e.g., a cellular phone or a pager, for reproducing an alarm sound responsive to a received call.
FIGS. 9A and 9B show a plan view and a cross-sectional view, respectively, of a conventional electroacoustic transducer of an electromagnetic type (hereinafter referred to as an xe2x80x9celectromagnetic transducerxe2x80x9d). The conventional electromagnetic transducer includes a cylindrical housing 107 and a disk-shaped yoke 106 disposed so as to cover the bottom face of the housing 107. A center pole 103, which may form an integral part of the yoke 106, is provided in a central portion of the yoke 106. A coil 104 is wound around the center pole 103. Spaced from the outer periphery of the coil 104 is provided an annular magnet 105, with an appropriate interspace maintained between the coil 104 and the inner periphery of the annular magnet 105 around the entire circumference thereof. The outer peripheral surface of the magnet 105 is abutted with the inner peripheral surface of the housing 107. An upper end of the housing 107 supports a first diaphragm 100 which is made of a non-magnetic disk so that an appropriate interspace exists between the first diaphragm 100 and the magnet 105, the coil 104, and the center pole 103. In a central portion of the first diaphragm 100, a second diaphragm 101 which is made of a magnetic disk is provided so as to be concentric with the first diaphragm 100.
Now, the operation and effects of the above-described conventional electromagnetic transducer will be described. In an initial state where no current flows through the coil 104, a magnetic path is formed by the magnet 105, the second diaphragm 101, the center pole 103, and the yoke 106. As a result, the second diaphragm 101 is attracted toward the magnet 105 and the center pole 103, up to a point of equilibrium with the elastic force of the first diaphragm 100. If an alternating current flows through the coil 104 in this initial state, an alternating magnetic field is generated in the aforementioned magnetic path, so that an driving force is generated on the second diaphragm 101. Such driving force generated on the second diaphragm 101 causes the second diaphragm 101 to vibrate from its initial state, along with the fixed first diaphragm 100, due to interaction with the attraction force which is generated by the magnet 105. This vibration is transmitted as sound. However, in the illustrated structure, the distance between the magnet 105 and the second diaphragm 101 is so large that the magnetic flux cannot sufficiently act on the second diaphragm 101.
FIG. 10 shows a magnetic flux vector diagram of the conventional electromagnetic transducer shown in FIGS. 9A and 9B. This magnetic flux vector diagram only illustrates one of the two halves with respect to a central axis (shown at the left of the figure), and the first diaphragm 100 and the housing 107 are omitted from illustration because they are non-magnetic. As seen from FIG. 10, a large magnetic gap exists in the magnetic path from the magnet 105 to the second diaphragm 101 of the conventional electromagnetic transducer. As a result, a large layer of air in the magnet gap serves as magnetic resistance, thereby making it difficult to supply sufficient magnetic flux from the magnetic path in the central portion of the magnet 105 to the second diaphragm 101.
It would seem possible to employ a first diaphragm 100 which is composed of a magnetic material so that the first diaphragm 100 can itself be utilized as a magnetic path. In this case, however, it would be difficult to form the first diaphragm 100 with a thickness which allows it to be utilized as a magnetic path while preventing magnetic saturation, especially if the first diaphragm 100 is designed so as to have a resonance frequency equal to the frequency which is intended to be reproduced as an alarm sound.
An electromagnetic transducer according to the present invention includes: a first diaphragm disposed in a vibratile manner; a second diaphragm provided in a central portion of the first diaphragm, the second diaphragm being formed of a magnetic material; a yoke disposed in a position opposing the first diaphragm; a center pole provided on a face of the yoke that opposes the first diaphragm; a coil substantially surrounding the center pole; a magnet substantially surrounding the coil; and a thin magnetic plate provided between the magnet and the first diaphragm, an inner periphery of the thin magnetic plate being in overlapping relation to an outer periphery of the second diaphragm.
In one embodiment of the invention, the first diaphragm, the magnet, and the yoke form an enclosed space.
In another embodiment of the invention, at least one of the first diaphragm, the magnet, and the yoke includes at least one air hole for allowing the enclosed space to communicate with the exterior of the enclosed space.
In still another embodiment of the invention, the electromagnetic transducer further includes a housing, the first diaphragm being provided in the housing.
In still another embodiment of the invention, the first diaphragm and the housing form an enclosed space.
In still another embodiment of the invention, at least one of the first diaphragm and the housing includes at least one air hole for allowing the enclosed space to communicate with the exterior of the enclosed space.
In still another embodiment of the invention, the first diaphragm, the housing, and the yoke form an enclosed space.
In still another embodiment of the invention, at least one of the first diaphragm, the housing, and the yoke includes at least one air hole for allowing the enclosed space to communicate with the exterior of the enclosed space.
In still another embodiment of the invention, the at least one air hole is provided in a position along a diameter of the yoke located outside an outer periphery of the magnet.
In still another embodiment of the invention, a length of radial overlap between an outer diameter of the second diaphragm and an inner diameter of the thin magnetic plate accounts for about 4% to about 15% of the outer diameter of the second diaphragm.
In still another embodiment of the invention, an inner diameter of the thin magnetic plate is equal to or smaller than an inner diameter of the magnet.
In still another embodiment of the invention, the magnet includes a recessed portion on a face thereof opposing the first diaphragm at an inner periphery thereof, the thin magnetic plate being snugly received by the recessed portion.
In still another embodiment of the invention, an outer periphery of the thin magnetic plate substantially coincides with a neutral point at which directions of magnetic flux vectors occurring on a surface of the magnet become diversified so that some of the magnetic flux vectors traverse toward the center pole while others traverse toward an outer periphery of the magnet.
In still another embodiment of the invention, the second diaphragm includes a plurality of projections, each of which extends in a radial direction, the plurality of projections being formed along a circumference direction of the second diaphragm.
In still another embodiment of the invention, a material substantially composing the first diaphragm has a specific gravity which is equal to or smaller than a specific gravity of a material substantially composing the second diaphragm.
In another aspect of the invention, there is provided a portable communication device incorporating any one of the aforementioned electromagnetic transducers.
Thus, the invention described herein makes possible the advantage of providing a high-performance electroacoustic transducer of an electromagnetic type in which a thin magnetic plate is provided between a magnet and a first diaphragm so as to complement the magnetic path between the magnet and a second diaphragm, thereby effectively generating attraction force and driving force on the second diaphragm, this being possible without substantial change in the size of the magnet and the second diaphragm.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.